Not sure I'm following this 100%.
If your OCPD can be a max of 60 amps why would you need larger than a #10 egc?
Because of what is written in 250.122(B). - but only if you run larger ungrounded conductors then typically needed, maybe for reasons like to correct voltage drop.
(B) Increased in Size.
Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.
So for our thread topic here with an AC unit - if the MOCP for the unit is 60 amps - chances are the MCA is going to be less then 60 so we are looking at probably 6AWG being the largest minimum sized conductor needed, but in many cases I bet it can be 8 AWG. 10 AWG EGC is all that T250.122 calls for on the EGC.
NOW, lets say for voltage drop reasons we increase that 6 AWG to 4AWG. This is a condition that makes 250.122(B) kick in. We now need to increase the EGC proportionally in size the same amount we increased the ungrounded conductors. The reason here is to ensure we didn't increase circuit impedance which ultimately will lessen the amount of fault current that does flow and will make it take more time for the overcurrent device to respond to that fault.
It is a silly rule to some extent though. I mean if we don't increase the 6 AWG to 4 AWG we likely have a higher circuit impedance then if we did increase to 4 AWG (which is why we did it to reduce voltage drop in the first place). It makes sense in some situations and doesn't make sense in others, they basically made a one rule fits all situation out of it.
So the 6 AWG to 4 AWG is an increase of 26,240 to 41,740 circular mils of conductor a 1 to 1.59 increase in size. The EGC needs to increase by the same ratio so multiply the circular mils of 10 AWG (10,380) by 1.59 and you get 16,504 circuilar mils needed. 8 AWG is 16510 - barely made it with 6 circular mils to spare or we would have had to go with 6 AWG.